1. Field of the Invention
The present invention relates to a method of manufacturing a positive electrode for a non-aqueous electrolyte battery, a slurry used therefor, and a non-aqueous electrolyte battery.
2. Description of Related Art
Lithium-ion secondary batteries achieve higher capacity and higher power relatively more easily than conventional secondary batteries. For this reason, the demands for lithium-ion secondary batteries have been increasing.
Many of the source materials for active materials of the lithium-ion secondary batteries have low durability against moisture. For this reason, slurries for forming electrodes have been prepared commonly using non-aqueous dispersion media, such as N-methylpyrrolidone (NMP), conventionally. However, the use of a non-aqueous dispersion medium raises the product cost of the slurry and increases also environmental impact. For this reason, the use of water as the dispersion medium has been desired since water has relatively low environmental impact and is available at low cost.
Graphite, for example, which is used as a negative electrode active material, is relatively stable against moisture. For this reason, aqueous dispersion media have been used for the negative electrode forming slurries containing graphite relatively early. Japanese Published Unexamined Patent Application No. 2003-272619 discloses a negative electrode forming slurry that uses an aqueous dispersion medium. Specifically, the just-mentioned patent application discloses that the pH of the negative electrode forming slurry is adjusted to 9 or higher using a pH adjuster.
On the other hand, positive electrode active materials are relatively unstable against moisture. For example, if water is used as the dispersion medium in the case that commonly used LiCoO2 is used as the positive electrode active material, lithium ions in the active material react with water, producing LiOH, for example. This results in corrosion of the current collector. As a consequence, the problem arises that battery performance deteriorates. Thus, it has been difficult to use an aqueous dispersion medium for a positive electrode forming slurry.
In view of this problem, the development of a positive electrode active material that is relatively stable against moisture has been underway in recent years, in order to make it possible to use an aqueous dispersion medium for the positive electrode forming slurry. For example, Japanese Published Unexamined Patent Application No. 2006-134777 discloses LixMPO4, where M is at least one kind of metal atom selected from the group consisting of Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, and Mo, and 0<x<2, as the positive electrode active material with relatively high stability against moisture.
In addition, the just-mentioned patent application discloses that a water-soluble polymer is added as a thickening agent to a positive electrode forming slurry using an aqueous dispersion medium. The addition of a thickening agent as described in the just-mentioned patent application can increase the viscosity of the slurry. As a result, coatability of the slurry can be improved. In addition, the just-mentioned patent application shows carboxymethylcellulose (hereinafter also referred to as “CMC”) as a specific example of the water-soluble polymer.
The present inventors have noticed, however, that dispersion stability, coatability, and adhesion strength of the positive electrode forming slurry become poor when CMC is used as a binder agent or a thickening agent for an aqueous slurry for a positive electrode active material such as lithium cobalt oxide. As a result of studies on the cause of this problem, the present inventors have found the following.
Lithium cobalt oxide is commonly produced by mixing a lithium source, such as a lithium hydroxide and a lithium carbonate, with a cobalt oxide, for example, and then sintering the mixture. When a lithium source and a cobalt oxide are mixed and sintered, the lithium source is usually added in excess of the cobalt oxide in order to prevent the cobalt oxide or the like from being left unreacted. As a consequence, the lithium source remains as a reaction residue.
Thus, lithium cobalt oxide generally shows mild alkalinity because of the lithium source added in an excess amount.
When CMC is used for an aqueous slurry in which such a lithium cobalt oxide is dispersed, sufficiently high viscosity cannot be obtained since CMC changes its viscosity greatly due to the pH of the slurry. This leads to the problems in dispersion capability, coatability, and adhesion strength.
It is an object of the present invention to provide a method of manufacturing a positive electrode for a non-aqueous electrolyte battery that is environmentally friendly and economical and that achieves excellent dispersion stability, coatability, and adhesion strength of the positive electrode forming slurry. It is also an object of the present invention to provide a slurry used therefor and to provide a non-aqueous electrolyte battery.